【作者】 朱彬；

【导师】 张宜生；

【作者基本信息】 华中科技大学 ， 材料加工工程， 2012， 博士

【摘要】 近年来,汽车行业在节能减排,提高安全性等需求的驱动下,高强钢越来越多的被用来制造汽车零部件。热成形技术由于可以获得超高的强度和良好的成形性能,作为一种新的成形制备工艺已经成为加工高强钢的主要方法之一。热成形工艺先将材料加热保温使其奥氏体化,之后成形-淬火同步完成,得到完全马氏体化组织,提高零件强度。结合数值模拟及试验对热成形工艺过程进行了分析。基于材料热力学理论、菲克定理和元胞自动机法,建立了高强钢奥氏体化的计算模型,开发了相关程序,模拟了高强钢在保温过程中,珠光体和铁素体组织向奥氏体转变以及奥氏体粗化和碳扩散的过程,并进行了试验验证。得出如下结论：(1)保温温度为950℃时,高强钢达到奥氏体转变开始温度后13s左右能够实现完全奥氏体化。此时碳浓度分布不均匀,奥氏体晶粒尺寸相差较大。奥氏体在铁素体中的生长同时受碳浓度和温度的影响,开始时奥氏体晶核碳浓度较高,生长速度较快；之后碳浓度降低,生长速度变慢；随着温度的继续升高,即使碳浓度较低也可使奥氏体在铁素体中快速生长。(2)本文所建立的模型所得到的结果与金属学基本理论相符,能够反映晶界曲率驱动下,奥氏体竞争生长的过程,得到了奥氏体粗化过程晶粒尺寸分布的规律。但由于模型中忽略了溶质拖拽、偏析等因素,计算结果偏大。基于马氏体相变动力学、传热学、热弹塑性理论建立了一种适用于有限元模拟的高强钢材料模型。该模型考虑了相变塑性、马氏体体积膨胀、热应力、相变潜热、多相组织共存对材料力学性能的影响。推导了相关公式,并开发了材料子函数,在通用有限元软件平台上,实现了高强钢热成形过程的多物理场模拟。研究结果表明：(1)利用新的材料模型对U形件热成形进行了模拟,发现U形件法兰部分冷却速度最快,侧边其次,底部冷却速度最慢,底部最有可能马氏体转变不充分。(2)利用新的材料模型及传统Johnson-Cook模型对U形件的厚度进行了预测。新材料模型比Johnson-Cook模型计算数值偏大更接近于实测值,原因在于新模型中考虑相变引发的马氏体膨胀。(3)结合数值模拟和试验分析了热成形回弹,推断马氏体体积膨胀是导致热成形与冷冲压回弹角度相反的原因。最后,制定了防撞梁高强钢热成形的工艺,完成了样品的试制,利用多物理场耦合模拟分析了其热成形加工过程。发现汽车门内防撞梁鼓包和凹窝连接处由于与模具接触时间晚,导致温度比邻近区域高,周围区域冷却收缩过程会对其产生一定的拉力,致使该区域减薄,影响了汽车门内防撞梁的成形精度。更多还原

【Abstract】 In recent years, for the demand of energy conservation and security improvement, high-strength steels are increasingly being used to produce the parts of an automobile. As an innovative process, hot stamping has become one of the main methods to process high-strength steel high strength because of its good formability and high strength obtained. In the hot stamping process, first the material was heated to obtain the uniform austenite and then form and quench the material at the same time.In this paper, analysis is made on the whole hot stamping process based on the numerical simulation and experiments.Based on the material thermodynamics theory and Fick diffusion Law, one cellular automata model is built to simulate the transformation process from ferrite and pearlite to austenite, the austenite coarsening and the carbon diffusion process of one HSS during the holding process in the furnace. The verification test is made after the simulation, and the conclusions are as follows:(1) The high strength steel blank could be fully austenitized within 13s after its temperature reached the austenite transformation start temperature, while it is heated in the furnace at constant 950℃. However, the carbon concentration distribution at this time is uneven, nor is the austenite grain size. High carbon concentration and high temperature can accelerate the growth rate of austenite grain. As a result, the growing up process shows a fast-slow- accelerating trend.(2) From the calculating results, which comply with the basic theory of metallography, we can get the austenite grain coarsening process motivated by grain boundary curvature growth and the distribution regulation of grain size. The ignorance of solute drag and the segregation could account for the lager calculating results.As for the hot stamping process, one material model suitable for the finite element analysis is built based on the martensitic transformation kinetics, heat transfer, and thermal elastic-plastic theory. The relevant formulas are derived and subroutines are developed to make multi- physics coupling simulation of the high strength steel hot stamping process. The simulation results are as follows:(1) The new constitutive model is used to predict the U-shaped hot-forming and the simulation result were coMPared with experimental results, and find that in the U-shaped parts the fastest cooling part is the flange, and the second is the side, while the cooling rate of the bottom is the slowest, so the austenite transformation is possibly not so sufficient at this part.(2) CoMPared with the Johnson-Cook model, the new constitutive model which takes the phase transformation-induced martensite expansion into consideration gives a larger calculation result and more close to the measured values.(3) Combination of the numerical simulation and experimental analysis about the hot stamping springback angle, it is found that the martensite volume expansion is the main reason of the opposite between the springback angle of hot stamping and cold forming.Finally, based on the above theoretical analysis, the process parameters of the hot stamping of Collision beam with high strength steel is developed and completed the sample trial, combined with the multi-physics coupling simulation analysis. The analysis results shows that because of the later contact with the mould, the temperature of the connection part between the bulge and dimple of the car door collision beams is higher than the neighboring areas, while the cooling and contraction of the surrounding area will produce a certain tension which result in the thinning of the region, and affects the car door anti-collision beam forming accuracy.更多还原